Radiation-induced inhibition of cell growth and proliferation will be studied, with particular emphasis on the deficiency in amount of DNA that is replicated in the generation following irradiation. HeLa S3 cells and 220 kV x-rays will be the main experimental materials. Treatment of irradiated cells with inhibitors of DNA synthesis and other compounds, in particular caffeine and its analogs, which results in enhanced cell killing, will be studied in further analysis of x-ray-induced potentially lethal damage.